It does nothing for me (Ubuntu Linux) here.
It does work on Windows.
What happens if you explicitly put keyboard focus on the instrument section?
Actually, what key does the preferences learn when you hit the numpad 0 while you have the cursor in the learn bar of the keyboard preferences for that key?
It learns the correct key, but the key doesn’t do anything, even if the instrument box is focused.
Switching numlock on/off doesn’t help either?
No, all the other numbers jump to the corresponding instruments, so numbers in general work, just the 0 doesn’t.
To clarify: This opens external VSTi or DSSI editors, not Renoises internal ones. Is this what doesn’t work for you on Linux?
If you reassign the shortcut, does it then work?
Aha! Mystery solved! Thanks! That could be clearer in the manual, though.